Project Summary An immense population of microorganisms, collectively referred to as the microbiota, colonizes the mammalian host. In addition to bacteria, nonpathogenic and pathogenic viruses enter hosts via the gastrointestinal tract. Of particular clinical importance are noroviruses, which are responsible for ~20% of pediatric diarrheal cases globally, are the leading cause of severe childhood diarrhea, and are associated with devastating infections in immunocompromised hosts. Considering that all enteric viruses encounter the intestinal microbiota as they traverse the mammalian gastrointestinal tract, it is imperative to consider how virus-bacteria interactions may impact the outcome of viral infections. Indeed, over the past seven years it has become well-established that commensal bacteria profoundly regulate enteric virus infections. The focus of this application will be the influence of the intestinal microbiota on noroviruses, using murine norovirus as a model system. We have recently discovered that commensal bacteria suppress norovirus infection of the proximal region of the gastrointestinal tract, and that this inhibition is entirely dependent on type III interferon signaling. Furthermore, we can rescue inhibition in bacteria-depleted mice by supplementing their chow with a single bile acid. These data lead to our working model that bacterially metabolized bile acids prime type III interferon induction in the proximal gut, thereby inhibiting norovirus infection. The main objective of this proposal is to delineate the underlying mechanism for bacterial suppression of norovirus infection in the proximal gut. To this end, we will test the specific hypothesis that primary bile acids suppress murine norovirus infection of the proximal gut; determine whether primary bile acids prime type III interferon induction in vivo; and uncover the molecular mechanism by which primary bile acids prime type III interferon production. Our findings could inform development of novel interventions based on prebiotics or applied metabolomics.